One-Liner:

Explaination:

awk '
NR==FNR {
## Loop through the file and check which line is repeated based on column 2
b[$2]++
## Skip the rest of the actions until complete file is scanned
next
}
## Once the scan is complete, look for second column in the array
$2 in b {
## If the count of the column is greater than 1 it means there is duplicate.
if (b[$2]>1) {
## So print that line with "acked" marker
print $0" acked"
## and delete the array so that it is not printed again
delete b[$2]
}
## If count is 1 it means there was no duplicate so print the line
else
print $0
}' inputFile inputFile